The present invention relates to a method of detecting land pre-pit (LPP) signals, in which a push-pull signal read from a digital video disk (DVD), which includes a groove and land pre-pits formed in the groove, is slice-shaped by a slice signal so as to detect LPP signals formed by binarizing LPP components, and an optical disk player performing said method.
The DVD is a data-writable DVD, e.g., DVD-R, DVD-RW, DVD+R, DVD+RW.
A phase changing material, whose phase changes between a crystal phase and a non-crystal phase, is used as a recording layer of the DVD. The recording layer is protected by a protection layer. A reflection layer is formed on the opposite side of the recording layer. The recording layer, the protection layer and the reflection layer are sandwiched between transparent plates, which are made of polycarbonate.
The recording layer of the DVD is shown in FIG. 7.
A groove 5 is spirally formed in the recording layer so as to guide a laser beam. A land 7 is spirally formed along the groove 5. The land 7 is projected from a surface of the recording layer. When the laser beam irradiates the surface of the recording layer, the phase changing material is phase-changed. The groove 5 is wobbled or meandered with a prescribed cycle. Time data can be given to an optical disk player on the basis of the cycle. Namely, the optical disk player detects wobble signals, which are based on the wobble groove 5, as time data, so as to control rotation of the disk.
Especially, in a DVD−R disk, DVD−RW disk, etc., land pre-pits (LPP) 9 are formed, as isolating pits, in the groove 5 at regular intervals. In the optical disk player, LPP signals having a prescribed cycle are used as time data for position control when data are written in the disk.
On the other hand, in an optical disk player for driving a DVD, an optical pick-up irradiate a laser beam toward the DVD and receives a beam reflected from the DVD. A light receiving element of the optical pick-up has a plurality of light receiving faces. Intensity of the reflected beam received by the light receiving faces are compared. A push-pull signal is generated on the basis of differences of the intensity of the reflected beam compared.
The push-pull signal is shown in FIG. 8.
The push-pull signal “a” is constituted by a wobble component “b”, whose amplitude and wave length correspond to the wobble or the meander of the groove 5, and pulse-shaped LPP components “c”, which correspond to the LPPs and which appears at regular intervals.
The LPP components “c” are projected from maximum or minimum peaks of the wobble component “b”, whose wave form is similar to a sine wave.
To extract the LPP components “c” from the push-pull signal “a”, signal levels are usually detected.
When the LPP components “c” exist at the peaks of the wobble component “b”, signals whose voltage is higher than specific signals, which have a prescribed voltage, are regarded as the LPP components “c”. The specific signals having the prescribed voltage are called slice signal. Namely, the push-pull signal “a” is compared with the slice signal, then the components “c” whose voltage are higher than the voltage of the slice signal is extracted as the LPP components “c”. The LPP components “c” are binarized as LPP signals. This method is disclosed in, for example, Japanese Patent Gazette No. 2003-123260.
To extract the LPP components “c” from the push-pull signal “a” with the slice signal, voltage of the slice signal must be defined previously.
In FIG. 9, voltage of the slice signal “A” is very close to peak voltage of the wobble component “b”. If the LPP components “c” is extract with the slice signal “A”, noises will be extracted together with the LPP components “c”. Therefore, it is difficult to securely extract only the LPP components “c” with the slice signal “A”.
On the other hand, voltage of the slice signal “B” is very close to maximum voltage of the LPP components “c”. If voltage of the slice signal “B” exceeds voltage of the LPP components “c”, the LPP components “c” cannot be extracted. Even if the LPP components “c” are extracted, their pulse width are very small so that the LPP components “c” cannot be securely extracted.
Therefore, voltage of the slice signal must be close to the peak voltage of the wobble component “b”, but it is difficult to securely extract the LPP components “c” without extracting noises.
Note that, in FIGS. 8 and 9, the LPP components “c” are projected from the maximum peaks of the wobble component “b”. In another case, the LPP components “c” are projected from the minimum peaks of the wobble component “b”. In this case, the voltage of the slice signal must be close to the minimum peak voltage of the wobble component “b”.